[hinkley]

It might be more of a commercial building thing than a domestic construction one. I know I've seen it in a number of videos of renovations of larger buildings, including barns. I might have the time range off. Big cities are full of 1920's constructions especially on the West Coast, and they don't do that.

My old house (~1920) had diagonal shiplap under the floors instead of plywood (but parallel in the oldest walls). That's probably more for making hardwood floors easier to install than structural integrity.

Edit: The internet says 'start of the 20th century' phased out in 1950's (plywood), and 'sometimes diagonally'.

[jefftk]

The subfloors I've seen for old houses in Boston are also laid perpendicular to the joists, though diagonal does seem like it would make more sense.

[hinkley]

Laying a board over a seam in the subfloor sounds like a titanic pain in the ass. Diagonal means you might have to move the nail a little. Probably also limits the amount of dust and water that passes through from floor to ceiling.